This can be either : your script (can you copy it ?) or the settings in tmgenc (did you choose "center" and nothing else ?)

###start
AviSource("C:\directory\movie.avi",false)
BlindPP(cpu=4)
Blockbuster(method="noise",detail_min=1,detail_max =3,variance=0.1,seed=1)
Convolution3D(1, 6, 12, 6, 8, 2.8, 0)
GripCrop(352, 480, overscan=1, source_anamorphic=true)
GripSize(resizer="LanczosResize")
Undot()
TemporalSoften(2,7,7,3,2)
DCTFilter(1,1,1,1,1,1,0.5,0)
#Blockbuster(method="noise",detail_min=1,detail_ma x=10,variance=0.3,seed=5623)
GripBorders()
###end

i didn't change any settings in tmpgenc. here are the defaults when i click settings:

size: 352*480
aspect ratio: 4:3 Display
23.976 fps

nvm, i think i solved my own problem.

i think i was experimenting with different settings and forgot to change back source_anamorphic to false.

An avi is never anamorphic ! (except digital camera DV format)